Photovoltaic cells convert electromagnetic energy (e.g., sunlight) into an electric current; accordingly, they are commonly used to provide electric power in a diverse range of applications. For example, photovoltaic cells may be incorporated in and provide electric power for devices as diverse as hand-held calculators and space vehicles.
Photovoltaic cells having a variety of characteristics have been developed. One class of photovoltaic cells that is currently the subject of significant research is the thin-film class. Thin-film photovoltaic cells include a plurality of layers of thin films formed on a substrate.
As mentioned above, photovoltaic cells (e.g., thin-film photovoltaic cells) may be operated from sunlight. Sunlight generally includes a plurality of colors of light, including light in the infrared and near ultraviolet bands, where each color of light has a different energy. For example, blue light has greater energy per photon than red light.
It is known that absorption of light in a photovoltaic cell typically requires that each photon have energy greater than the bandgap energy of the absorbing material, such that carriers can be excited into the free or nearly free carrier bands (conduction and valence) in the absorbing material. These carriers then move to appropriate sides of the junction, providing an output voltage and current. Absorbed photon energy in excess of this bandgap energy is usually wasted in that additional carriers are not always generated. Semiconductor materials that have electrons as the dominant free carriers are defined as n-type materials, and materials that have holes, or the absence of an electron, as the dominant free carriers are defined as p-type materials.
The maximum output voltage of the cell is related to the bandgap of the materials used to form the junction. For example, higher bandgap materials can provide higher output voltages. As a result, photovoltaic cells optimized to absorb long wavelength, or low energy, photons will also absorb short-wavelength photons, but will waste much energy by producing a lower output voltage than attainable with cells optimized to absorb the short wavelength, high energy, photons.
A photovoltaic cell optimized to absorb short wavelength, e.g. blue, light will fail to efficiently absorb long wavelength, or red and infrared, light because the bandgap of the material is greater than the available energy in each photon, and therefore, photons are not absorbed and carriers are not promoted into the conduction.